Receiver circuits for millimeter wave wireless communication systems typically include a low noise amplifier (LNA) and a down-converter for converting the received, high frequency signal to a lower, intermediate frequency (IF) signal, from which the baseband signal is subsequently extracted. Monolithic microwave integrated circuit (MMIC) fabrication technology enables the low noise amplifier and down-converter mixer circuits to be formed on a single integrated circuit chip, in order to improve performance, simplify production and reduce costs. For relatively low microwave carrier frequencies, the down-converter may employ a fundamental mixer, in which the same frequency generated by the local oscillator (LO) is mixed with the received RF signal to generate the intermediate frequency components. At low microwave frequencies, the local oscillator frequency is normally well separated from the RF frequency so that the LO signal can be readily isolated from the RF mixer port using standard filtering techniques. However, at higher microwave frequencies, the frequency of the local oscillator signal becomes geometrically closer to the RF carrier frequency in order to down-convert the RF frequency to a suitable intermediate frequency. This proximity of the local oscillator frequency to the RF frequency can lead to undesirable LO frequency radiation, and also isolation problems between the RF and LO mixer ports. Another drawback of using fundamental mixers in microwave frequency applications is that the local oscillators required to provide adequate output power at millimeter wave frequencies tend to be relatively bulky and expensive.
An alternative form of mixer which is employed in microwave frequency receivers is the sub-harmonic mixer, in which the injected mixer frequency which is mixed with the RF frequency is a multiple of the LO frequency generated by the local oscillator. This arrangement provides better frequency separation between the local oscillator and RF frequencies, thereby making it easier to prevent LO frequency signals leaking into the RF port and to prevent RF frequency signals passing to the LO port.
An example of a known sub-harmonic mixer comprises a pair of field effect transistors having interconnected drains and interconnected sources, and a 180° hybrid coupler having in-phase and anti-phase signal output ports coupled to a respective gate of the FET combination, and an input port for receiving a local oscillator signal. The RF and IF ports are coupled to the FET drains.